Knotmeters or knotlogs as they are sometimes called have been provided in the past in which water movement past an impeller provides a signal related to the impeller speed and thus the flow of water past the impeller. As is common, impeller speed may be readily converted into an indication of boat speed, either through the magnitude of a signal generated by a generator coupled to the impeller or by the number of pulses per second generated when a magnetic element in the impeller passes by a Hall effect transistor. In general, the latter type of knotmeter transducers generate between 17,000 and 24,000 pulses per knot.
One of the difficulties with the utilization of an impeller type knotmeter is that the impeller can become fouled both by barnacles and marine growth, but more importantly and more recently by eel grass, weeds, and in fact plastic bags and debris in the water. In the past the problems of fouling the impeller have been addressed through housings which permit the removal of the impeller portion of the sensing unit. At the same time it is necessary to simultaneously put a plug in the housing to stop the gush of water as the impeller element is removed from the housing since this housing is in the form of a conduit which passes through the hull of the boat. This of course is both time consuming, inconvenient, and sometimes dangerous in that if the plug is not properly positioned, the boat can sink. Invariably, however, the individual performing the cleaning operation gets soaking wet.
The frequency of clearing the impeller has increased in recent years such that on a given boat trip, the impeller may need to be cleaned under way as often as every three or four miles. This is an intolerable situation and one which renders the knotmeter useless for most of the time.
Additionally, for those boats utilizing transom mount transducers, the impeller may be damaged by virtue of a log or floating debris physically hitting the impeller and bending or damaging it so that it will no longer rotate. Moreover, for those transducers having a flip up capability, the knotmeter ceases to function when the transducer is flipped up, requiring reaching over the stern of the boat to flip the transducer down to its operating position.
All of the above militates against a properly functioning knotmeter. Additionally, speed through water is of course no indication of speed over the bottom.